Efficient laser material machining is based on the optimization of the interaction of laser light with matter for a plurality of different processes.
For example, it is known that, when thin metal sheets are cut, a small focus having a small depth of focus allows higher cutting speeds and cutting quality than a large focus. In case of thicker sheets having thicknesses of more than about 6 mm, a larger focus is required for efficient cutting. A wider kerf has to be generated to be able to efficiently expel the liquefied material, whereas a narrow kerf is sufficient with thin sheets. In order to be able to quickly adjust the focus to different requirements, special cutting optics have been developed which allow the mechanical displacement of lenses. Further, it is known that different materials have a different interaction with the laser and thus an adjustment of the focus to the material is required, too.
In laser welding, it is also known that the geometry of the focus has a large influence on the geometry and quality of the welding seam. For example, in fast welding of typically more than 15 m/min, humping is observed. In this case, the welding bead forms major irregularities. An elongated beam increases the maximum welding speed by up to the factor two or more. Typically, welding optics are used that generate two foci which can be adjusted with respect to their position to one another and their respective intensity.
Further, the focus geometry has to be matched with the thickness of the sheets to be welded and the existent geometry of the joining edges. This is obvious for the butt joint. When both joining edges have a rectangular geometry, this results in a minimum joining gap, and a small focus achieves fast welding speeds and better quality. But if the joining gaps are poorly prepared, the result is a large gap, and a broad focus has to be used. For the lap weld, it is known that a large depth of focus and a small focus exhibit a high gap bridging ability whereas large foci and a small depth of focus can only bridge small gap widths. In the case of known edge quality and geometry special optical systems are used, respectively. Gap bridging is achieved, for example, by hybrid optics which combines MIG welding (metal inert gas welding) and laser welding or introduces wire as additional material into the laser beam. Scanners are used, too, which rapidly oscillate the laser beam and thus melt additional material which serves for bridging gaps.
Different material thicknesses in the butt joint require a dedicated beam shaping in order to make the heat input into the joining zone symmetrical, to prevent an overheating of the thin sheet, and to heat the thick sheet correspondingly. Here, double beam optics is often used, as described above.
Due to intermetallic phases, the welding of different materials poses a challenge, too. Here, the specific heat input and the temperature distribution developing between the join partners are crucial for the welding quality. For example, when aluminum and copper are joined, most of the beam intensity is applied on the copper in order to compensate for its thermal conductivity and its higher melting point.
Another example for beam shaping can be found in joining zones of different widths, for example, in the lap weld of plastics. Fine geometries and flat connections are found in many components. The absolute heat input and the line energy have to be exactly controlled to prevent burning of the plastic at the edge. Here, an exposure by masks or scanners is often used which generate a focus line adapted to the seam geometry.
Welding of galvanized sheets poses a challenge because the zinc evaporates in the intermediate layer during laser welding and outgasses through the melt, wherein big and irregular bubbles occur in the welding seam, which negatively influence the stability. It has been shown that special and complex focus geometries can largely prevent outgassing. Here too, double beam optics is used, but often more complex focus geometries are required.
In the field of build-up welding, a plurality of different focus geometries are used. Filigree structures with high surface finish require small spots and performance whereas large surfaces typically are coated with large round or line foci. Here, mostly fixed optics with dedicated focus is used.
Similarly, this is the case for laser hardening where the focus and the power have to be dynamically adapted to the component to prevent melting of the skin layer. Special fixed optics as well as scanners are used here to select the focus correspondingly or to adapt it to the tool geometry.
Ablating processes and drilling also benefit from a dynamic variation of the focus and the power. For example, different structures or any hole geometries can be easily ablated correspondingly. Mostly, a scanner is used for this purpose.
In addition to the required flexibility in spot geometry and power of single processes of the laser material machining, the execution of several processes with an optics is desirable in order to optimize the utilization of the system. However, the requirements on the focus geometry and intensity are even more diverse than in the case of single processes, and thus this is not possible with today's optics.
Commercial optical systems are very limited in their flexibility to vary the focus geometry. Twin spot optics with a variable distance of two foci and their intensity, microlens arrays for homogenizing the intensity mainly in conjunction with the generation of line foci and diffractive optical elements for generating a determined arbitrary pattern in connection with low performance (FIG. 1) are known. All these optics have in common that they generate a determined focus geometry and only—if at all—allow a small flexibility of the focus geometry. Thus, an adaptation to the varying requirements of the processes is only possible with an enormous amount of time and money.
Scanners direct a laser beam over two movable mirrors and subsequently focus it with a f-theta lens. By means of this arrangement, a focused laser beam can be moved over the work piece at 10 m/s and faster. Specific geometries are programmed. Hereby, different focus geometries can be adjusted in a very flexible manner. However, the fast control between the laser and the scanner poses a technical challenge. So, it is only limitedly possible to dynamically adapt the power to the process. For a good spatial resolution, regulation times of a few microseconds are required, but they typically amount to tens to hundreds of microseconds. But during this time the focus may be already moved a few tens of millimeters over the work piece.
Further, a scanner is a complex and expensive optical unit and also far away from the work piece, and thus the introduction of additional materials represents a considerable additional effort.